Animation Mixamo's FBX in blender
I had this problem and managed to fix it by doing this:

First you need to scale your model correctly to the size you want in Blender. Then while in object mode press A to select all and then press Ctrl+A and apply scale, rotation and location.
After that, export your model as an .obj.

Import it into Mixamo, animate it and then export it as a .dae.

Now, import the .dae into blender. Re-scale the model to what you had it as before and then it should work.